The present invention relates to a safety access port structure for a furnace or other fired vessel, such as a coal gassifier. Access ports on coal gassifiers have been used for viewing the interior of the vessel and for poking or stirring the bed of coal and clinkers in the vessel. Particular difficulty and danger ar encountered by operators if the access port is not protected or shielded when it is opened. Ports of the prior art have included jets of steam or other fluid injected tangentially of the port to create a vortex which prevents the fluids within the vessel from flowing through the open port.
One problem which has been encountered with prior access ports on coal gassifiers is than even though a steam vortex creating jet is provided it is possible to open the access port without having the steam jet in operation.